WURSIG and WUESIG: BEHAVIOR AND ECOLOGY OF THE DUSKY DOLPHIN 



I 

 h- 



a. 



UJ 



Dam. 

 tJPM, 



0500- 1200 



1200 - 1900 186 



559 



•444 82 f^ 



326 887 





0( 



4 



A 



121 



i 





AUG SEPT OCT NOV DEC JAN FEB 



MONTHS 



AM, 0500 - 1200 

 PM, 1200 - 1900 



20 - 



E 



— 10 

 Q 

 UJ 

 LJ 

 CL 



i 



A 



X-sa. 



i [^ 





* 



*■ 



ID 



4 



AUG SEPT OCT NOV DEC JAN FEB 



MONTHS 



Figure 9. — Mean depth of water (a) and mean speed (b) of dusky 

 dolphin travel in mornings versus afternoons, separated into 

 those months for which adequate data are available. The lines 

 above and below bars represent 95% confidence intervals for 

 means, and numbers represent number of theodolite readings. 

 During August and September, dolphins were -found in sig- 

 nificantly shallower water than during the spring and summer 

 months of October-February I Mann-Whitney {/-test. P< 0.001). 

 During August and September, dolphins also moved sig- 

 nificantly slower than October through January (Mann- 

 Whitney (/-test, P<0.001) Almost all speed increases in these 

 months took place in the afternoon. 



they moved with no feeding bouts present in the 

 area, and as they moved near feeding bouts (Table 

 1). The mean speed without feeding bouts was 6.3 

 km/h, while speeds around feeding bouts averaged 

 about 15 km/h. 



Dusky dolphins spent more time in deeper water 



when surface feeding. Furthermore, the depth of 

 water in which surface feeding occurred increased 

 as the summer season advanced (Figure 10). Thus 

 the mean depth of feeding bouts during September 

 was 21m, but by February dolphins were surface 

 feeding in waters 41m deep. Since it is a general 

 rule (and confirmed for Golfo San Jose by Pizzaro 

 ( 1976), and pers. obs. ) that deeper offshore water is 

 cooler than shallow nearshore water in summer, 



SEPT OCT NCW DEC JAN FEB 



MONTHS 



Figure id. — Mean depth of dusky dolphin surface feeding bouts 

 during different months. Bars above and below means represent 

 95'7c confidence intervals for means. Numbers above bars repre- 

 sent the number of theodolite readings of feeding bouts per 

 month. 



this change in preferred feeding locations may 

 represent a change in movement patterns of fish 

 upon which the dolphins were feeding. We caught 

 fish from schools on which the dolphins were feed- 

 ing on 15 separate occasions, and identified the 

 species composing such schools in the field about 

 50 more times. In all cases, the fish were southern 

 anchovy, Engraulis anchoita. These fish are found 

 in deeper water during summer in a nearby coast- 

 al area, where they are netted by fishermen 

 (Mermoz^). and we suspect that they move into 

 deeper water in summer in the present study area 

 as well. 



*J. Mermoz, research scientist, Museo de Ciencias, Buenos 

 Aires, Argentina, pers. commun. 1975. 



877 



